signal encoding_a.doc

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Signal Encoding Techniques

Both analog and digital information can be encoded as either analog or digital signals:

Digital data- digital signals: simplest form of digital encoding of digital data

Digital data- analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog

Analog data- digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission

facilities

Analog data- analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different

frequency band, which can be utilized on an analog transmission system

Fig below emphasizes the process involved in this. Fordigital signaling, a data sourceg(t), which may be either digital or analog, is

encoded into a digital signalx(t). The basis foranalog signaling is a continuous constant-frequencyfc signal known as the carrier

signal. Data may be transmitted using a carrier signal by modulation, which is the process of encoding source data onto the carrier

signal. All modulation techniques involve operation on one or more of the three fundamental frequency domain parameters: amplitude,

frequency, and phase. The input signal m(t) may be analog or digital and is called the modulating signal, and the result of modulating

the carrier signal is called the modulated signals(t).

Encoding - Digital data to digital signals:

A digital signal is a sequence of discrete, discontinuous voltage pulses, as illustrated in Figure below. Each pulse is a signal element.

Binary data are transmitted by encoding each data bit into signal elements. In the simplest case, there is a one-to-one correspondence

between bits and signal elements. More complex encoding schemes are used to improve performance, by altering the spectrum of the

signal and providing synchronization capability. In general, the equipment for encoding digital data into a digital signal is less complex

and less expensive than digital-to-analog modulation equipment.


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Before discussing this further, we need to define some terms:

Unipolar - All signal elements have the same sign

Polar - One logic state represented by positive voltage the other by negative voltage

Data rate - Rate of data (R) transmission in bits per second

Duration or length of a bit - Time taken for transmitter to emit the bit (1/R)

Modulation rate -Rate at which the signal level changes, measured in baud = signal elements per second. Depends on type of digitalencoding used.

Mark and Space - Binary 1 and Binary 0 respectively.

Interpreting Signals

The tasks involved in interpreting digital signals at the receiver can be summarized as follows. First, the receiver must know the timing

of each bit, knowing with some accuracy when a bit begins and ends. Second, the receiver must determine whether the signal level for

each bit position is high (0) or low (1). These tasks can be performed by sampling each bit position in the middle of the interval and

comparing the value to a threshold. Because of noise and other impairments, there will be errors. As was shown in Chapter 3, three

factors are important: the signal-to-noise ratio, the data rate, and the bandwidth. With other factors held constant, the following

statements are true:

An increase in data rate increases bit error rate (BER).

An increase in SNR decreases bit error rate.

An increase in bandwidth allows an increase in data rate.

There is another factor that can be used to improve performance, and that is the encoding scheme. The encoding scheme is simply the

mapping from data bits to signal elements. A variety of approaches have been tried. In what follows, we describe some of the more

common ones.

Comparison of Encoding Schemes

Before describing the various encoding techniques, consider the following ways of evaluating or comparing them:

Signal Spectrum - Lack of high frequencies reduces required bandwidth, lack of dc component allows ac coupling via

transformer, providing isolation, should concentrate power in the middle of the bandwidth

Clocking - need for synchronizing transmitter and receiver either with an external clock or with a sync mechanism based on

signal

Error detection - useful if can be built in to signal encoding

Signal interference and noise immunity - some codes are better than others

Cost and complexity - Higher signal rate (& thus data rate) lead to higher costs, some codes require signal rate greater than data rate

Encoding Schemes

We now turn to a discussion of various techniques, They include:

Nonreturn to Zero-Level (NRZ-L)

Nonreturn to Zero Inverted (NRZI)

Bipolar -AMI

Pseudoternary

Manchester

Differential Manchester

B8ZS

HDB3


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Non-return to Zero-Level (NRZ-L)

The most common, and easiest, way to transmit digital signals is to use two different voltage levels for the two binary digits. Codes that

follow this strategy share the property that the voltage level is constant during a bit interval; there is no transition (no return to a zero

voltage level). Can have absence of voltage used to represent binary 0, with a constant positive voltage used to represent binary 1.

More commonly a negative voltage represents one binary value and a positive voltage represents the other. This is known as

Nonreturn to Zero-Level (NRZ-L). NRZ-L is typically the code used to generate or interpret digital data by terminals and other

devices.

Non-return to Zero Inverted

A variation of NRZ is known as NRZI (Nonreturn to Zero, invert on ones). As with NRZ-L, NRZI maintains a constant voltage pulse

for the duration of a bit time. The data bits are encoded as the presence or absence of a signal transition at the beginning of the bit time.

A transition (low to high or high to low) at the beginning of a bit time denotes a binary 1 for that bit time; no transition indicates a

binary 0.

NRZI is an example ofdifferential encoding. In differential encoding, the information to be transmitted is represented in

terms of the changes between successive signal elements rather than the signal elements themselves. The encoding of the current bit is

determined as follows: if the current bit is a binary 0, then the current bit is encoded with the same signal as the preceding bit; if the

current bit is a binary 1, then the current bit is encoded with a different signal than the preceding bit.

One benefit of differential encoding is that it may be more reliable to detect a transition in the presence of noise than to compare a

value to a threshold. Another benefit is that with a complex transmission layout, it is easy to lose the sense of the polarity of the signal.

NRZ Pros & Cons


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The NRZ codes are the easiest to engineer and, in addition, make efficient use of bandwidth. Most of the energy in NRZ and NRZI

signals is between dc and half the bit rate. The main limitations of NRZ signals are the presence of a dc component and the lack of

synchronization capability. Consider that with a long string of 1s or 0s for NRZ-L or a long string of 0s for NRZI, the output is a

constant voltage over a long period of time. Under these circumstances, any drift between the clocks of transmitter and receiver will

result in loss of synchronization between the two.

Because of their simplicity and relatively low frequency response characteristics, NRZ codes are commonly used for digital

magnetic recording. However, their limitations make these codes unattractive for signal transmission applications.

Multilevel Binary Bipolar-AMI

A category of encoding techniques known as multilevel binary addresses some of the deficiencies of the NRZ codes. These codes use

more than two signal levels. Two examples of this scheme was illustrated in Figure above.

In the bipolar-AMI scheme, a binary 0 is represented by no line signal, and a binary 1 is represented by a positive or negative

pulse. The binary 1 pulses must alternate in polarity. There are several advantages to this approach. First, there will be no loss of

synchronization if a long string of 1s occurs. Each 1 introduces a transition, and the receiver can resynchronize on that transition. A

long string of 0s would still be a problem. Second, because the 1 signals alternate in voltage from positive to negative, there is no net dc

component. Also, the bandwidth of the resulting signal is considerably less than the bandwidth for NRZ. Finally, the pulse alternation

property provides a simple means of error detection. Any isolated error, whether it deletes a pulse or adds a pulse, causes a violation of

this property.

Multilevel Binary Issues

Although a degree of synchronization is provided with these codes, a long string of 0s in the case of AMI or 1s in the case of

pseudoternary still presents a problem. Several techniques have been used to address this deficiency. One approach is to insert

additional bits that force transitions. This technique is used in ISDN (integrated services digital network) for relatively low data rate

transmission. Of course, at a high data rate, this scheme is expensive, because it results in an increase in an already high signal

transmission rate. To deal with this problem at high data rates, a technique that involves scrambling the data is used.

Manchester Encoding

There is another set of coding techniques, grouped under the term biphase, that overcomes the limitations of NRZ codes. Two of these

techniques, Manchester and differential Manchester, are in common use.

In the Manchester code, there is a transition at the middle of each bit period. The midbit transition serves as a clocking mechanism and

also as data: a low-to-high transition represents a 1, and a high-to-low transition represents a 0. Biphase codes are popular techniques

for data transmission. The more common Manchester code has been specified for the IEEE 802.3 (Ethernet) standard for baseband

coaxial cable and twisted-pair bus LANs.

In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 is represented by the presence

of a transition at the beginning of a bit period, and a 1 is represented by the absence of a transition at the beginning of a bit period.

Differential Manchester has the added advantage of employing differential encoding.

Differential Manchester has been specified for the IEEE 802.5 token ring LAN, using shielded twisted pair.


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Biphase Pros and Cons

All of the biphase techniques require at least one transition per bit time and may have as many as two transitions. Thus, the maximum

modulation rate is twice that for NRZ; this means that the bandwidth required is correspondingly greater. The bandwidth for biphase

codes is reasonably narrow and contains no dc component. However, it is wider than the bandwidth for the multilevel binary codes.

On the other hand, the biphase schemes have several advantages:

Synchronization: Because there is a predictable transition during each bit time, the receiver can synchronize on that transition, known

as self-clocking codes.

No dc component: Biphase codes have no dc component

Error detection: The absence of an expected transition can be used to detect errors. Noise on the line would have to invert both the

signal before and after the expected transition to cause an undetected error.

Digital Data- Analog Signal

We turn now to the case of transmitting digital data using analog signals. The most familiar use of this transformation is for

transmitting digital data through the public telephone network. The telephone network was designed to receive, switch, and transmit

analog signals in the voice-frequency range of about 300 to 3400 Hz. It is not at present suitable for handling digital signals from the

subscriber locations (although this is beginning to change). Thus digital devices are attached to the network via a modem (modulator-

demodulator), which converts digital data to analog signals, and vice versa.

Have stated that modulation involves operation on one or more of the three characteristics of a carrier signal: amplitude, frequency, and

phase. Accordingly, there are three basic encoding or modulation techniques for transforming digital data into analog signals, as

illustrated in Figure below:

amplitude shift keying (ASK),

frequency shift keying (FSK), and

phase shift keying (PSK).

In all these cases, the resulting signal occupies a bandwidth centered on the carrier frequency.


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Amplitude Shift Keying

In ASK, the two binary values are represented by two different amplitudes of the carrier frequency. Commonly, one of the amplitudes

is zero; that is, one binary digit is represented by the presence, at constant amplitude, of the carrier, the other by the absence of the

carrier, as shown in Figure a below.

ASK is susceptible to sudden gain changes and is a rather inefficient modulation technique. On voice-grade lines, it is

typically used only up to 1200 bps.

The ASK technique is used to transmit digital data over optical fiber, where one signal element is represented by a light pulse

while the other signal element is represented by the absence of light.

Binary Frequency Shift Keying

The most common form of FSK is binary FSK (BFSK), in which the two binary values are represented by two different frequencies

near the carrier frequency, as shown in Figure b below.

BFSK is less susceptible to error than ASK. On voice-grade lines, it is typically used up to 1200 bps. It is also commonly used

for high-frequency (3 to 30 MHz) radio transmission. It can also be used at even higher frequencies on local area networks that use

coaxial cable.


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Multiple FSK

A signal that is more bandwidth efficient, but also more susceptible to error, is multiple FSK (MFSK), in which more than two

frequencies are used. In this case each signaling element represents more than one bit. To match the data rate of the input bit stream,

each output signal element is held for a period ofTs =LTseconds, where Tis the bit period (data rate = 1/T). Thus, one signal element,

which is a constant-frequency tone, encodesL bits. The total bandwidth required is 2Mfd. It can be shown that the minimum frequency

separation required is 2fd= 1/Ts. Therefore, the modulator requires a bandwidth ofWd= 2Mfd =M/Ts.

Phase Shift Keying

In PSK, the phase of the carrier signal is shifted to represent data. The simplest scheme uses two phases to represent the two binary

digits (Figure c below) and is known as binary phase shift keying.

An alternative form of two-level PSK is differential PSK (DPSK). In this scheme, a binary 0 is represented by sending a signal

burst of the same phase as the previous signal burst sent. A binary 1 is represented by sending a signal burst of opposite phase to the

preceding one. This term differentialrefers to the fact that the phase shift is with reference to the previous bit transmitted rather than to

some constant reference signal. In differential encoding, the information to be transmitted is represented in terms of the changes

between successive data symbols rather than the signal elements themselves. DPSK avoids the requirement for an accurate local

oscillator phase at the receiver that is matched with the transmitter. As long as the preceding phase is received correctly, the phase

reference is accurate.

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